The present invention relates to the treatment of mercury containing waste. More particularly, the invention relates to a process for treating mercury containing waste by stabilizing and encapsulating the waste through the use of sulfur polymer cement.
At the present time a great deal of effort is being expended for finding economic and effective methods of disposing of hazardous waste, including mercury containing waste, so that it does not leach into the soil or water supply. Such waste often contains radionuclides, making its handling and disposal even more problematic. This highly variable type of waste includes radioactively contaminated elemental mercury, as well as bulk materials or debris that contain dispersed mercury.
Large amounts of such mercury containing waste has been generated by both military and civilian uses. Much of this waste is xe2x80x9cmixed wastexe2x80x9d, i.e., it also contains radionuclides. The Department of Energy has estimated that it has greater than 38,000 m3 of mixed low-level and transuranic mercury containing waste throughout various facilities in the U.S. Approximately 6 m3 of liquid elemental mercury are currently being stored and additional inventories are expected to be generated.
Congress has enacted the Resource Conservation and Recovery Act (RCRA) mandating cleanup and disposal of hazardous materials, including toxic metals such as mercury. With respect to mercury containing waste, the Environmental Protection Agency (EPA) requires that the mercury be combined with reagents such as copper, gold, or sulfur that result in a solid, non-volatile product. RCRA requires that the waste form pass the Toxicity Characteristic Leaching Procedure (TCLP) in order for the mercury containing waste treated according to EPA regulations to be disposed of properly.
Waste containing dispersed mercury is sometimes treated by separation methods, such as retorting and/or extraction. The separation processes will remove the mercury from the bulk waste. However, a secondary waste stream of concentrated mercury is generated, which then must be disposed of. Also, retorting (heating with recovery of mercury for reuse) is not appropriate for radioactive waste.
Other methods of treating such waste involve encapsulation. In such methods, an effort is made to seal the mercury waste from the surrounding environment. For example, in U.S. Pat. No. 5,569,153 a method is disclosed in which pozzolana, a calcium hydroxide containing material, hydrothermal cement reactant, hydraulic cement, and toxic waste are mixed with additional liquid to form a moldable mixture, which is then cured. The cured form is then embedded into a sulfur cement. While encapsulation methods have reduced the leachability of the mercury in the waste, they have not been proven to be a solution to long-term storage or disposal.
Attempts have also been made to react mercury within the waste directly with inorganic sulfur compounds to form mercury sulfide, which is an insoluble compound. However such processes have proven impractical due to dangerous gases or dust formed during such processes. For example, U.S. Pat. No. 4,354,942 discloses a method for stabilizing mercury-containing materials by directly adding selected inorganic sulfur compounds to prevent the leaching of soluble mercury therefrom. However, the use of inorganic sulfur compounds alone, often results in inadvertent mixing of acids with the sulfides, thereby releasing toxic hydrogen sulfide gas. Also, the use of inorganic sulfur compounds to directly react with mercury can be prohibitively expensive. For example, sodium sulfide presently costs about $2.50/lb whereas sulfur polymer cement, the main ingredient in the process of the invention, presently costs approximately $0.12/lb. Thus, if inorganic sulfur compounds are going to be used, it is much more cost effective to utilize them as an additive.
The direct reaction of elemental mercury and elemental sulfur to form HgS has also been studied. See Gorin, A. H. et al. xe2x80x9cFinal Disposal Options for Mercury/Uranium Mixed Wastes From The Oak Ridge Reservation,xe2x80x9d Y/DZ-1106, August 1994. This method is shown to have better leaching properties and lower vapor pressures compared with other reagents. This treatment produces a chemically stable dry powder, but does not provide any additional barrier to leaching. Further, the resulting dry powder is susceptible to mechanical dispersion, which is particularly troublesome when the waste mix is radioactive.
Gorin et al. also disclose the possibility of reacting the mercury with a modified sulfur cement, also known as sulfur polymer cement (SPC). However, they conclude that this method is inferior to the direct reaction of elemental sulfur because of the engineering difficulties, the excessive process temperature required, and unacceptable leaching characteristics of the resulting samples.
Thus, there remains a need for methods of stabilizing and solidifying mercury containing waste so that it can be stored or disposed of without presenting a pollution problem.
The present invention provides a process for the treatment of mercury containing waste in a single reaction vessel. The process is effective in treating various types of mercury contaminated waste; such as elemental mercury or mercury compounds, mercury contaminated bulk material, or mercury contaminated debris. The process is also effective for the treatment of mercury containing waste that also contains radionuclides, i.e. mixed wastes.
The first step of the process (step (a)) is the stabilization of the mercury containing waste. This stabilization step includes combining mercury containing waste with sulfur polymer cement under an inert atmosphere to form a resulting mixture. Argon or nitrogen are preferred. The second step of the process (step (b)) is the encapsulation of the waste. In the encapsulation step, the resulting mixture of step (a) is heated to form a molten product, which is then cast as a monolithic final waste form.
The preferred weight ratio of SPC to mercury containing waste in step (a) is typically between about 0.2 to about 3.0, preferably about 1.0. The initial combining of mercury containing waste with the sulfur polymer cement can occur at a preferred temperature range of from about 20xc2x0 C. to about 80xc2x0 C. To facilitate the reaction, it is preferred that the sulfur polymer cement be reduced to a particle size of less than about 3000 microns, before being added as a reagent.
In a preferred embodiment, a stabilizing additive is further added to the mercury containing waste and the sulfur polymer cement. The stabilizing additives can be sodium sulfide, triisobutyl phosphine sulfide, calcium hydroxide, sodium hydroxide, calcium oxide, and magnesium oxide, or a combination thereof. Sodium sulfide or triisobutyl phosphine sulfide or a combination thereof are preferred. Sodium sulfide is most preferred. It is preferred that the stabilizing additive also be of a particle size less than about 3000 microns, when added as a reagent. The amount of stabilizing additive is usually between 0.5 and 20 wt % of the molten product, preferably between about 1-12 wt %, and most preferably between 2-5 wt %. When sodium sulfide is added as the stabilizing additive, the most preferred range is from about 2.0 wt % to about 3.0 wt % of the final waste form.
In the encapsulation step, the resulting mixture of step (a) is heated to a temperature of usually between approximately 120xc2x0 C.-150xc2x0 C. to form a molten product. If necessary, additional SPC can be added to the resulting mixture of step (a) to improve the viscosity of the molten product to allow for more homogenous mixing. If additional SPC is added, it is added to the resulting mixture of step (a) so as to form a waste loading of usually about 5 wt % to about 90 wt % in the final waste form. If the mercury containing waste is essentially mercury contaminated bulk materials or debris and additional SPC is added, the additional SPC is usually added in an amount to form a waste loading of about 25 wt % to about 80 wt % in the final waste form, preferably about 50 wt % to about 70 wt %. If the mercury containing waste is essentially elemental mercury or mercury compounds and additional SPC is added, the additional SPC is usually added in an amount to form a waste loading of about 10 wt % to about 50 wt % in the final waste form, preferably about 30 wt % to about 35 wt %.
The invention also includes a waste form comprising mercury containing waste stabilized and encapsulated by the addition of sulfur polymer cement and a stabilizing additive.
The process of the invention for treating mercury containing waste, therefore, produces a waste form which can more safely be disposed of, for example, in a landfill or licensed disposal facility. The process reduces mercury solubility in the waste form to enable compliance with EPA TCLP criteria, lowers mercury vapor pressure during processing and in the final product, eliminates dispersibility of the stabilized product, and reduces leachability of radioactive constituents within the waste.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.